The field of the invention is that of fiber optic telecommunications. The invention relates to a photosensitive fiber for fabricating a Bragg grating filter and to a method of fabricating a fiber of this kind. The filter obtained using this fiber is particularly suitable for compensating chromatic dispersion and chromatic dispersion slope effects in a fiber optic link without introducing any penalty in terms of polarization mode dispersion.
The effects of chromatic dispersion are cumulative over the length of the link and are therefore greater on long links. Also, chromatic dispersion causes temporal widening of the pulses transmitted over the link. The risk of errors at the receiver due to this widening is lower if the pulses are sufficiently spaced in time. However, in very high bit rate systems the temporal widening can be comparable with the spacing between pulses, leading to the possibility of an error rate unacceptable to the link operator.
In a practical link, as deployed at present, another cause of pulse widening is polarization mode dispersion (PMD).
It is therefore important to minimize PMD and to correct chromatic dispersion periodically in long-haul high bit rate links.
The prior art, in particular document D1=Laming, R. I. et al., xe2x80x9cDispersion compensating fiber Bragg gratingsxe2x80x9d, Proc. WFOPCxe2x80x94Workshop on Fiber Optics Passive Components, University of Pavia, Sep. 18-19, 1998, pp. 108-116, includes a fiber Bragg grating (FBG) which is xe2x80x9cchirpedxe2x80x9d to compensate chromatic dispersion. The following documents may also be referred to on the subject of dispersion compensation:
D2=Loh, W. H. et al.,: xe2x80x9cDispersion compensated 10 Gbit/s transmission over 700 km of standard single mode fiber with 10 cm chirped fiber grating and duobinary transmitterxe2x80x9d, Proc. OFC ""96, paper PD 30, San Jose Calif., USA, 1996.
D3=Cole, M. C. et al., xe2x80x9cBroadband dispersion compensating chirped fiber Bragg gratings in 10 Gbit/s NRZ 100 km non-dispersion shifted fiber link operating at 1.5 xcexcmxe2x80x9d, Electron. Lett. V.33 (1) pp. 70-71, 1977.
Document D1 teaches the use of a quadratically chirped FBG to compensate dispersion slope (see p.114). To create the Bragg grating with the required characteristics within the fiber, the core is first charged with a dopant which renders the fiber photosensitive. Refractive index variations are then induced along the fiber by irradiating the fiber with ultraviolet (UV) light through a phase mask, using a standard method. The fabrication parameters of interest are therefore:
The composition and concentration of the photosensitive dopants;
The wavelength, luminous intensity (power) and exposure time of the ultraviolet light; and
The form (contrast, spacing, etc.) of the phase mask.
According to document D1, the phase mask is uniform and the wavelength reflected by the Bragg grating is modified according to the length of the fiber in the region to be exposed. Linear variation of the wavelength along the fiber can be used to correct second order chromatic dispersion (conventionally referred to as chromatic dispersion). Quadratic variation of the phase (inversely proportional to the wavelength) along the length of the fiber can be used to correct third order chromatic dispersion (dispersion slope).
However, the method taught by document D1 is difficult to implement and would seem to be ill-suited to an industrial scale process. Furthermore, a fine structure of group time variations with wavelength has been observed. Uncontrolled effects such as fluctuations of the fiber and mechanical vibrations cause quasi-interferometric noise. Despite low polarization dependent differential losses (PDL), differential group delay dispersion (PMDxe2x80x94polarization mode dispersion) has been observed, attributed by the authors to birefringence of the host fiber. This birefringence is attributable either to deformations of the fiber (stresses, ellipticity, etc.) or to the UV radiation.
Document D4=Williams, J. A. R. et al., xe2x80x9cFiber Bragg Grating fabrication for dispersion slope compensationxe2x80x9d, IEEE Photonics Tech. Lett. 8 (9) pp. 1187-1189, September 1996, teaches another method of providing a dispersion slope corrector. According to document D4, a photosensitive fiber is irradiated in two steps: first a constant pitch linear grating is inscribed optically in the core of the fiber using a uniform phase mask; a second irradiation is then performed at constant power, without a mask and at a speed that is varied along the fiber. The object of varying the speed is to vary the exposure time and therefore the luminous energy delivered, in order to modify the optically induced index variation. The result is that the mean index varies along the fiber and the Bragg wavelength therefore varies in the same manner along the fiber. This method has yielded a chromatic dispersion slope corrector device with a narrow bandwidth, in the order of 0.7 nm. However, technical process control problems arise in implementing devices having a greater usable bandwidth: aligning and focusing the laser onto the core of the fiber, controlling the acceleration and rate of displacement of the fiber, etc.
An object of the invention is to alleviate the known problems of the prior art in providing a Bragg grating dispersion slope corrector filter and the performance problems of filters obtained in this way.
To this end, the invention proposes a photosensitive optical fiber for forming an optically induced Bragg grating filter, said optical fiber having a core and a cladding, said core of said fiber having a refractive index nc and said cladding of said fiber having a refractive index ng, said core being doped with a photosensitive first element, and said first element being Germanium, wherein the concentration CGe of Ge in the core is at least equal to 10%: CGexe2x89xa710%, and wherein said core is further doped with a photosensitive or non-photosensitive second element to reduce the refractive index nc of the core to reduce the refractive index difference to a value less than: xcex94n=ncxe2x88x92ngxe2x89xa66xc3x9710xe2x88x923. This low refractive index step is to minimize the unwanted effect of PMD.
The invention further proposes a Bragg filter formed from a photosensitive optical fiber of the above kind in which said Bragg grating is formed in the core of an optical fiber as a quasi-periodic succession of variations of the optical refractive index xcex94n1 along the length (z) of said fiber with a period close to xcex94L; said Bragg grating also having a variation xcex4L(z) of said period xcex94L along the length (z) of said fiber; wherein said variation xcex4L(z) of said period xcex94L is a linear or quadratic variation along the length (z) of said fiber. Linear variation can correct at least second order chromatic dispersion; quadratic variation can correct third order chromatic dispersion (dispersion slope).
According to a feature of the invention the optical refractive index variations xcex94n1 are high: xcex94n1xe2x89xa73xc3x9710xe2x88x924. According to another feature of the invention said variation xcex4L(z) of said period xcex94L is a linear variation along the length (z) of said fiber.
Linear variation of the period, with high index variations, corrects third order chromatic dispersion (dispersion slope).
In a variant of the invention, the cladding of said fiber is also doped with said first photosensitive element Ge. According to another feature of the invention a third photosensitive or non-photosensitive element is added to the cladding to reduce the index difference xcex94n=ncxe2x88x92ngxe2x89xa66xc3x9710xe2x88x923, again with the aim of obtaining low PMD. In various embodiments of the invention:
said second doping element is fluorine;
said second doping element is boron;
said third doping element is fluorine;
said third doping element is boron.